Note 7

The Human Body

If we are to understand the universe, we must first understand ourselves; and the first element of that is to understand our bodies. McMillan’s work presents a stunning collection of photographs accompanied by clear explanations of this wonderful machine.

Below is a compendium of cogent (or are they useless?) facts relating to the human body; some of these points everyone knows, others might not be so well known. Nevertheless, this corpus (pun intended) of information should go a long way in providing to the reader an acquaintance that should be common knowledge about ourselves to ourselves at our most fundamental level.

Genes comprise only 1% of human DNA. For a time it was a mystery how about 24,000 genes were sufficient to build a human being. However, 7.2% of DNA scientists once believed was “junk,” and now recognize has a major impact on our development. This involves the discipline of epigenetics i.e. activities that modify gene expression. Some segments function as control “switches” to turn genes on and off, others affect gene splicing or encode the production of important compounds. But the remaining 91.8% of our DNA is junk.

Human DNA is at least 5% derived from retroviruses. These viruses hijack the cell’s machinery for decoding DNA into protein to reproduce themselves. Sometime in evolution, they assimilated into human DNA, and now carry out important tasks in shaping our genomic expression.

The fundamental unit of life is the cell, and the body produces about 232 billionnew ones each day!

Skeletomuscular system

Human bone is five times stronger than steel but five times lighter!

At birth, we possess over 300 bones. Later various bones start to fuse and consequently an adult has only 206 bones.

The thighbone is the lengthiest bone in the body; it’s about a quarter of a person’s stature.

Each hand has twenty-seven bones while each foot has twenty-six.

The body has over 640 muscles.

Muscles come in three varieties: skeletal (which work in pairs—one to move the bone one-way and the other to move it back), smooth (which has the power to stretch and sustain tension for long periods), and cardiac (located only in the heart, which combines traits from the other two kinds).

We sit on our biggest muscle: the gluteus maximus.

The smallest muscle, the stapedius of the middle ear, is only one mm long.

Nervous System

Nerve Impulses move up to 119 m/sec.

The spinal cord, which has about a billion nerve cells, measures around 44 cm and has a diameter roughly that of the index finger.

The brain contains over 100 billion nerve cells, and these have about one quadrillion connections between them. But any damage to nerve cells is never completely repaired.

The typical brain weighs around 1,360 grams, and its surface area is around 25,000 cm2.

The brain totals 60% white matter, which holds essential structures such as the thalamus and hypothalamus, and aids in transmitting sensory data from the body to the cerebral cortex. It controls autonomic functions, such as heart rate and body temperature, and certain parts play a role in limbic system functioning that involves the release of pituitary hormones, and the control of eating. The remaining 40% is grey matter, which comprises specialized areas involving attention, memory, perceptual cognizance, thought, language, and consciousness. Though 20% of the entire oxygen we breathe travels right to the brain, 95% of that diffuses into the gray matter.

The left hemisphere of the brain operates the right side of the body and the right hemisphere of the brain operates the left side. These hemispheres are intricately interconnected, differentiated only by their processing methods. The concept that people are “right brain thinkers” or “left brain thinkers” is a myth.

The eye sees an inverted image. The brain takes this, flips it over leaving a normal image, and fills in the blind spot i.e. the zone of the retina where the optic nerve connects, which lacks sensors.

The conscious mind includes sensations, perceptions, feelings, memories, and thoughts within our present cognizance. The preconscious mind contains thoughts that are unconscious at any particular moment but can move easily into consciousness. The subconscious mind is all mental activity that works below consciousness.

In one year, an average adult sleeps for 114 days. When we sleep and move into the REM (Rapid Eye Movement) phase, the body becomes entirely paralyzed as parts of the brain that control movement disengage.

A child’s learning aptitude can increase or decrease by 25% or more subject to whether the child matures in a stimulating environment. Reading aloud to them aids in stimulating brain development.

The classification of drugs on the nervous system (from mild to strong) is summarized by the following table:

Scientists call the eye’s light sensing receptors “rods” and “cones.” About 120 million rods record the shapes of images and react in dim light. Cones react to bright light, record the color of images, and number about six million.

The ear can hear between the frequencies of 20 to 20,000 hertz.

The inner ear is the principal organ of balance.

The nose is the body’s air-conditioning system: it heats cold air, cools hot air, and filters out impurities.

The sense of smell has the capability of distinguishing the chemical odor of a compound in one part per trillion of air.

If the sense of smell is off, the sense of taste is also off as the brain processes signals from the nose and tongue together.

The tongue contains 10,000 taste buds.

The body has five kinds of touch sensations: cold, heat, contact, pressure, and pain. The fingertips possess the largest amount of touch nerve endings.

Circulatory and Respiratory Systems

Blood is composed of red blood cells transporting oxygen, white blood cells that fight infection, platelets that aid in clotting the blood and a liquid called plasma (about 55% of blood volume) comprising water (95% of plasma), sugar, fat, protein, and salts.

The body has around 5.5 liters of blood, which circulates throughout it every twenty seconds!

Why is blood red? Iron attaches to a ring of atoms in hemoglobin known as porphyrin and the shape of this structure creates its color. The degree of redness is contingent on whether there is oxygen bound to the hemoglobin. When oxygen is bound, it alters the shape of the porphyrin, imparting to the red blood cell a brighter shade of red.

Red blood cells make up about 40% of blood volume.

To sustain the blood’s red blood cell count, the bone marrow must produce around two and a half million new ones every second.

A red blood cell lives for about 120 days.

Red blood cells are the only cells in the body that lack a nucleus.

The bone marrow produces one hundred billion platelets daily, which make up about 4% of blood volume.

The average adult’s heart pumps over 7,500 liters of blood daily.

The aorta, the biggest artery in the body, is two to three cm thick. Capillaries, conversely, are so tiny that it takes ten of them to equal the breadth of a human hair.

Each lung has 300 million respiratory components called alveoli.

A person breathes over seven liters of air each minute.

At rest, the average adult breathes around twelve to twenty times a minute.

Yawning brings in more oxygen to the lungs.

The lungs excrete 350 ml of water a day through breathing.

Immune System

There is more bacterial life inside the body than human. Ten percent of the body’s dry weight derives from bacteria. There are about 37.2 trillion body cells, but around 370 trillion bacteria. However, many are helpful.

Inflammation is an initial response of the immune system to attack. Typical symptoms include redness, swelling, pain, heat, and loss of function. At the site of inflammation, immune cells migrate to repair damage and hasten the healing process.

About 7,000 white blood cells, aka lymphocytes, live in every microliter of blood. Their job is to manage the immune response. They comprise around 1% of blood by volume.

The lymphatic system is a network of vessels that gathers interstitial fluid from the spaces separating cells. Before it returns to the veins, this fluid travels to lymph nodes to kill any possible pathogens that it might contain. These nodes, including the thymus, spleen, and tonsils, are “lymphoid tissues,” indicating that they contain lymphocytes.

We are more liable to “catch cold” (over 100 viruses are capable of doing so) from shaking a person’s hand than from their sneeze or their cough.

We “catch cold” rather easily during the winter (in the Northern Hemisphere) since we are more often indoors in a confined space.

Digestive System

Humans consume around 500 kg of food each year.

Teeth are virtually as hard as rock.

Plaque starts to develop one hour following a teeth cleaning.

The body produces about 1.6 liters of saliva per day.

The esophagus, the passageway where the food/water we consume travels to the stomach, is about 25 cm long.

Food will reach the stomach even if one stands on one’s head!

Every four days the stomach generates a new lining, or else it will digest itself.

The stomach has around 35,000,000 small digestive glands.

The typical stomach stores around two liters but can swell to four especially after Thanksgiving dinner!

The liver is the largest and heaviest internal organ and weighs about 1.5 kg.

Three-quarters of the liver’s cells can fail before it stops functioning; said another way, the liver is the only organ in the body that can completely regenerate itself with only 25% of the organ remaining.

The small intestine is around six meters long and the large intestine is around one and a half meters long. “Small” and “large” are their respective diameters.

The appendix is enormously helpful to the good bacteria that aid in the digestive system’s operation. They use it to reproduce which assists in the maintenance of the gut’s healthy balance.

Endocrine System

The body contains fifty incredible hormones, which regulate activities like sleep, temperature, hunger, and the management of stress etc.

The eight hormone-secreting glands forming the endocrine system include the pituitary, pineal, hypothalamus, thyroid, parathyroid, pancreas, adrenal, and testes/ovaries. They are consigned the job of controlling metabolic functions, growth, sleep, reproduction, etc.

Typically, human growth hormone, which causes a person’s growth among other functions, is released at the rate of 500 micrograms a day at the age of twenty, 200 micrograms a day at forty, and 25 micrograms a day at eighty.

Reproduction

The body’s largest cell is the egg/ ovum produced in the ovaries and its smallest is the sperm produced in the testes.

There are about 200-500 million sperm released when a normal adult male ejaculates.

The ovaries contain about 300,000 eggs at puberty.

Upon reaching her sixties, a woman will have ovulated about 450 baby-making eggs.

The lifespan of an egg is about 12-24 hours. Sperm, on the other hand, are able to live for up to five days within a woman’s body.

Pregnancy on average lasts about 270 days between conception and birth.

Each one of us spent about half an hour after fertilization as a single cell.

A fetus develops fingerprints by the age of three months, and even identical twins don’t share them.

Urinary System

A person can survive deprived of food for around a month, but only a week deprived of water.

A typical person requires about 2.2-3 liters of water per day.

All the blood in the body passes 400 times through each kidney every day.

Each kidney contains about one million nephrons that filter out extra fluid and wastes.

A fully filled bladder can store 500 ml of urine.

Skin, Hair, and Nails

Varying with which area of the body, the thickness of skin differs from ½ to 14 mm.

An average adult’s skin weighs between 3.6-4.5 kg.

The body sloughs off about 600,000 particles of skin per hour—around 680 grams a year. Skin cells reproduce every second to replenish the ones lost.

Melanin, a pigment found in skin, hair, and eyes, causes their color. Carotene also affects skin color.

Hair is the fastest growing tissue in the body, followed by bone marrow.

A hair grows 0.3 to 0.5 mm a day, 1 to 1.5 cm a month and 12 to 15 cm a year.

On a typical scalp, there are around 100,000 hairs, of which are lost about 50-100 a day.

“There is,” writes Jessica Krant, M.D. “a huge amount that a well-trained dermatologist can tell about your overall health just by examining nails. From nail bed discoloration (blueish means lung disease), to capillaries in the cuticles (autoimmune disease), to yellow, white, or banded nails, sometimes very serious or even life-threatening disease can be diagnosed just by examining the tips of your fingers. . . .”

Fingernails and toenails require around 6 months growing time from base to tip.

Homeostasis

The body controls its temperature by neural feedback mechanisms that function largely through the hypothalamus. This comprises not only the control processes, but also the major temperature sensors.

Shivering is a method of preserving the body’s warm.

Between two and four million sweat glands are contained in an adult’s body.

The palms and soles of the feet have more sweat glands than any other part of the body.

A hard working adult perspires up to fourteen liters a day. The majority of it evaporates before a person realizes it’s even there.

Why did humans lose their protective fur? Perhaps it may have been to aid them perspire more easily, or to deny a home for parasites such as lice and ticks. However, the most appealing proposal is that they required a greater degree of cooperation when they left life in the trees and moved into the savanna. When animals are bred to maximize cooperation, as was once done with wolves to generate dogs, they develop more like their infant forms. Humans correspondingly have certain traits of infantile apes—big heads, little mouths, and most meaningfully in this context, thinner body hair.

Goosebumps are a vestige from our evolutionary ancestors. They result when miniscule muscles at the bottom of each hair stiffen, tugging the hair upward. With a sufficient coating of fur, this would ruffle up the coat, moving more air into it, and allowing it to make a better insulator. Many mammals ruff up their fur when fearful, and humans still experience the sensation of hairs standing on end when frightened.